1. Field of the Invention
The present invention relates to an optically active compound having plural chiral centers and processes for producing said compound. More particularly it relates to an optically active 2-substituted-3-substituted-carboxylic acid ester and an optically active 2,6-cis-2,5,6-substituted-1,3-dioxan-4-one and processes for producing the compound.
2. Description of Related Art
Optically active compounds, in particular, optically active 2-substituted-3-hydroxycarboxylic acid esters and their esters, are considered useful as medicines, starting materials or intermediates for producing biologically active compounds such as medicines, agricultural chemicals, and the like and functional materials.
However, such optically active compounds have two chiral centers resulting in the presence of four stereo-isomers and therefore, it is necessary for exhibiting sufficient biological activities or sufficient functions that only one stereoisomer is present in a large amount. Consequently, efficiently separating the four stereo-isomers from one another is a big problem.
In order to produce optically active compounds, for example, an asymmetric synthesis is carried out; racemic compounds produced by ordinary chemical synthesis are subjected to optical resolution; or an optically active compound is converted to another desired optically active compound by a stereochemical method.
In particular, optically active methyl 2-methyl-3-hydroxybutanoate can be produced by firstly forming the salt and then resolving it according to recrystallization [A. Tai and M. Imaida, Bull. Chem. Soc. Jpn., 51, 1114 (1978)], or ethyl (2R, 3R)-2-propyl-3-hydroxypentanoate can be produced by alkylation with lithium diisopropylamide (hereinafter called "LDA") [G. Frater, Helv. Chim. Acta., 62, 2829 (1979)].
It is difficult to carry out in an industrial scale the above-mentioned method where the salt is first prepared followed by recrystallization, and moreover only a limited number of compounds can be resolved by recrystallization. The alkylation with LDA requires a reaction condition of extremely low temperatures (-78.degree. C. to -50.degree. C.) and, in addition, an optically active 3-hydroxycarboxylic acid ester as a starting materials. Further, in the above-mentioned reaction, only the anti-form can be obtained and the selectivity is not 100%, but anti-form: syn-form=94:6.
Some 1,3-dioxan-4-one compounds have already been prepared and the cis/trans selectivity is utilized for studying the synthesis of optically active secondary alcohols and the like [S. L. Schreiber et al., Tetrahedron Lett., 27, 2945 (1986) ; D. Seebach et al, Angew. Chem. Int. Ed. Engl., 25, 178 (1986)].
However, there is little searching for optically active compounds having a substituent at the 5-position. The only one is that where the unsubstituted carbon atom at the 5-position is alkylated by using LDA [D. Seebach, Helv. Chim. Acta., 69, 1147 (1986)], but LDA has drawbacks that, as mentioned above, it should be used in a reaction at extremely low temperatures such as, for example, -75.degree. C. and only one enantiomer can be obtained.